platform/x86: surface3-wmi: Shut up unused-function warning

The newly added driver guards its "resume" callback with an
warning in some configurations:

drivers/platform/x86/surface3-wmi.c:248:12: error: 's3_wmi_resume' defined but not used [-Werror=unused-function]

Using a __maybe_unused annotation without an #ifdef avoids the mistake more
reliably.

Fixes: 3dda3b37 ("platform/x86: Add custom surface3 platform device for controlling LID")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>